The mechanism of water tree growth in XLPE cables based on the finite element method

To study the mechanism of water tree growth in the cross-linked polyethylene (XLPE) cables, liquid needle electrode and high-frequency high-pressure were used for water tree accelerated aging in XLPE slice samples, and the shapes of water trees were observed by microscope after heating in silicone oil. The function of electrical-mechanical stress during the initiation and growth of water tree was studied by building water tree electric field simulation model and finite element analysis. The relationship of water tree growing and electrical-mechanical stress was analyzed by building the mathematical model. The results show that electrical-mechanical stress is the main cause of water tree growth. The moisture in environment is gathered at the impurities or defects of cable insulation and the water filled voids are formed under the action of alternating electric field. The alternating Maxwell stress is applied on the XLPE materials surrounding of water filled voids, which causes the rupture of molecular chain. The accumulation of fatigue fracture results in the increasing of the volume and number of micro voids. The micro voids are connected by water tree channels, and the water tree morphology is formed.